It's nothing to do with the ISP. The Domain Name Registrar must provide as part of the registration process access to some form of domain name management. That's what they do.
Typically they provide the ability to specify the IP address of the DNS server on which the primary domain name source records exist.
If your friend is the registered owner of the domain name then the registar is required to provide access to enable the owner to change the DNS Server associated with the domain name.